ABSTRACT
How plants obtained their nutrition undoubtedly puzzled farmers and other observers of the countryside in ancient, and also early historical, times. Up to the rst half of the nineteenth century the idea largely prevailed that soil humus, that is, the organic component of the soil, was one of the plants’ key nutriments and supplied them with part of the carbon they needed, the soil minerals hence being not really fertilizers but only contributing indirectly to their growth. This was indeed Thaer’s opinion (Feller et al. 2003), which has been identied as the “humus theory.” Based on several previous investigations, Liebig (1840) reversed this conception in showing that the plant nutriments withdrawn from the soil are minerals originating in fact from degraded organic matter or from the soil’s mineral constituents and that all of the plants’ organic carbon is exclusively derived from air or soil carbon dioxide and not from humus. Liebig’s (1840) publication actually produced an abrupt shift of paradigm as regards views on the nutrition of plants. Indeed, this new “mineral theory” of plant nutrition opened up the era of mineral and chemical fertilization (Boulaine 1989, 1992).
